U.S. Pat. No. 3,767,756 describes a process for spinning para-aramids to provide filaments having excellent as-spun tenacity, modulus and breaking elongation. In practice, sulfuric acid having a concentration of at least 98% is used as the spinning solvent. Due to the degrading effects of even small amounts of acid in the yarn, complete removal of the acid is very important in obtaining high tenacity fibers. Water alone or combinations of alkaline solutions and water have been used for its removal. The sulfuric acid is partly removed from the spinning threadline in a coagulation bath whereby most of the sulfuric acid is removed. As disclosed in U.S. Pat. No. 4,048,279, the acid containing yarn is further washed first with a 0.3 to 1.3% aqueous caustic solution and then with a more dilute aqueous caustic solution having a concentration of 0.01 to 0.1%.
The para-aramid filaments obtained by the above-described spinning and washing processes have excellent properties but even further improvement in tenacity and modulus is desired. From U.S. Pat. No. 3,869,429 it is known that drying the filaments obtained using such processes under tensions above 0.3 gpd reduces the breaking elongation of the filaments while increasing the modulus. Drying under tension is also disclosed in U.S. Pat. No. 3,869,430, Example IIe. No effect of drying tension on the tenacity of the filaments or yarns thereof has been reported.
Japanese Application Publication Kokai 53-98415, published Aug. 28, 1978, owned by Asahi Chemical Industry Co., Ltd., also describes the increase in modulus resulting when para-aramid yarns are dried under tension to low moisture contents. With only one exception, all the examples show drying to less than 4 percent by weight water. The single exception is its Practical Example 4 to a moisture content of 5.1%. Their stated goal is to "substantially increase modulus without causing heat deterioration, while maintaining intrinsic viscosity and tensile strength . . . ".
In typical washing and drying processes using yarn tensions of less than 1 gpd, the amount of moisture remaining on the yarn after drying has been shown to have little effect on the yarn tenacity.